Hey fellow Final Fantasy X explorer! When it comes to sphere grid progression, I recommend focusing on unlocking key abilities for each character first before branching out. Prioritize upgrading your main stats like HP, MP, Strength, and Defense to make battles easier.

For side quests, don’t miss out on the Celestial Weapons and Aeons. They require some effort to obtain but are totally worth it for the endgame. Also, make sure to complete the Monster Arena challenges for some great rewards and tough battles.

When it comes to character builds, tailor each character to their roles. For example, Tidus excels in speed and agility, so focus on boosting his agility and strength. Yuna is a great healer, so enhance her magic and MP. Experiment with different paths on the sphere grid to create unique and powerful builds for each party member. Good luck on your journey!